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Unit 7 Economics and Transportation EngineeringText An economic analysis should not be carried out as all afterthought.It should be used as part of a continuous process，starting with the objectives of the proposed transportation project (can these objectives be satisfied in any other way?)，running through the entire planning and design process，and only ending，as a final summing up，with the overall evaluation. It should consider the following questions：Project identification. Which projects or policies should be considered as possible solutions to a particular transport need?Establishing rough prioritiesWhich projects and policies should be considered in detail?Detailed evaluationWhich costs and benefits are relevant to the evaluation，how should they be measured，and how can they best be presented as an index of priority?Project selectionShould this project be selected，should it be rejected，or should it be postponed?Economics is often overlooked as an aid to transportation planning and designYet any transportation engineering problem involves a whole series of essentially economic decisionsIn the area of design，for example，the transportation engineer must choose materials，select an overall design concept，and then combine the component parts of the scheme into an effective，and economical，wholeA river crossing could thus be by ferry boat，causeway，or bridgeIn the latter case it could be in steel，concrete，or timberIt could be suspended，arched，or simply supportedWhat should the engineer choose?The range of choice is not usually as large as thisSome solutions will be physically infeasible (e.g.，by lack of suitable foundations for an arched bridge), but important choices will remainIn the absence of a rigorous economic analysis，the designer may make these choices on the basis of：Experience，which may or may not cover an adequate range of optionsPreconceptions as to what is desirable (e.g., one solution uses less material)An innate view of mathematical or structural elegance (e.g.，an arch is a more elegant structural form)Economics avoids the need for these arbitrary rules by providing a tangible criterion-cost for choosing between alternative solutions. In many cases this can be done quite simply by drawing envelopes of cost curves of, for example, steel girder bridges versus reinforced concrete bridges versus prestressed concrete bridges. By plotting cost (y) against span length (x), the engineer can choose which type is most economic over a given range of span lengths.In practice, the engineer does not usually carry the economic analysis to this level of detail, although curves like this commonly form part of the standard highway design repertoire. It is more a question of the economic attitude of mind that asks: is this the only possible solution, and if not, is there a better one? It is this general principle which eventually ensures that the final design is the most economical and the one which, if justified in aggregate terms, offers best value for money.Economic efficiency and distributionEconomics is not concerned solely with the optimum allocation of resources, but with wider questions of equity and the distribution of costs and benefits among individuals, regions, etc. It is thus concerned with the question: who does what, to whom, and at whose expense? In a sense this concern complements the intertemporal considerations noted above. A dollar to one person is not necessarily worth the same amount to another. Since economics assumes that marginal values decline as income increases, interpersonal differences in income may thus affect any overall measure of consumer benefit. Questions of distribution nevertheless go beyond mere differences in income. Institutional constraints usually prevent beneficiaries, defined in the broadest sense, from compensating people who are adversely affected. It is therefore often appropriate to separate the impact of a transport improvement into its effect on different interest groups, in addition to its effect on different income groups. Notes1Some solutions will be physically infeasible(eg，by lack of suitable foundations for an arched bridge)，but important choices will remain一些解决方法实际上是不可行的(例如一座拱桥缺乏适当的基础)，但仍会有一些重要的选择将被保留。2Economics avoids the need for these arbitrary rules by providing a tangible criterion-cost-for choosing between alternative solutions经济学通过在选择备选方案时提出一个切实的成本标准，从而避免随意性规则的使用。3.It is more a question of the economic attitude of mind that asks：Is this the only possible solution,and if not,is there a better one?这更多的是一个从经济学角度提出的问题：这是唯一可能的解决方法吗，如果不是，还有更好的方法吗? 4Questions of distribution nevertheless go beyond mere differences in income不过分配问题超过收人上的微弱差别。5It is therefore often appropriate to separate the impact of a transport improvement into its effect on different interest groupsin addition to its effect on different income groups因此，除了影响不同的收入阶层外，通常将交通改善的影响还按其对不同的利益阶层的影响来加以区分。Unit 8 Sight DistanceText The ability to see ahead is of the utmost important in the safe and efficient operation of a vehicle on a highway. On a railroad，trains are confined to a fixed path，yet a block signal system and trained operators are necessary for safe operationOn the other hand，the path and speed of motor vehicles on highways and streets are subject to the control of drivers whose ability，training，and experience are quite variedFor safety on highways the designers must provide sight distance of sufficient length that drivers can control the operation of their vehicles to avoid striking an unexpected object on the traveled wayCertain two-lane highways should also have sufficient sight distance to enable drivers to occupy the opposing traffic lane without hazardTwo-lane rural highways should generally provide such passing sight distance at frequent intervals and for substantial portions of their lengthConversely，it normally is of little practical value to provide passing sight distance on two-lane urban streets or arterialsThe length and interval of passing sight distance should be compatible with the criteria established in the chapter pertaining to that specific highway or street classification Sight distance is discussed in four steps：the distances required for stopping，applicable on all highways；the distances required for the passing of overtaken vehicles，applicable only on two-lane highways；the distances needed for decisions at complex locations；and the criteria for measuring these distances for use in designStopping sight distance Sight distance is the length of roadway ahead visible to the driverThe minimum sight distance available on a roadway should be sufficiently long to enable a vehicle traveling at or near the design speed to stop before reaching a stationary object in its pathAlthough greater length is desirable，sight distance at every point along the highway should be at least that required for a below average operator or vehicle to stop in this distance Stopping sight distance is the sum of two distances：the distance traversed by the vehicle from the instant the driver sights an object necessitating a stop to the instant the brakes are applied and the distance required to stop the vehicle from the brake application beginsThese are referred to as brakes reaction distance and braking distance，respectively Design sight distance Stopping sight distances are usually sufficient to allow reasonably competent and alert drivers to come to a hurried stop under ordinary circumstances. However, these distances are often inadequate when drivers must make complex or instantaneous decisions, when information is difficult to perceive, or when unexpected or unusual maneuvers are required. Limiting sight distances to those provided for stopping may also preclude drivers from performing evasive maneuver，which are often less hazardous and otherwise preferable to stopping. Stopping sight distances may not provide sufficient visibility distances for drivers to corroborate advance warnings and to perform the necessary maneuvers. It is evident that there are many locations where it would be prudent to provide longer sight distances. In these circumstances, decision sight distance provides the greater length that drivers need. Decision sight distance is the distance required for a driver to detect an unexpected or otherwise difficult-to-perceive information source or hazard in a roadway environment that may be visually cluttered, recognize the hazard or its threat potential, select an appropriate speed and path, and initiate and complete the required safety maneuver safely and efficiently. Because decision sight distance gives drivers additional margin for error and affords them sufficient length to maneuver their vehicles at the same or reduced speed rather than to just stop, its values are substantially greater than stopping sight distance.Passing sight distance for two-lane highways Most roads and numerous streets are considered to qualify as two-lane two-way highways on which vehicles frequently overtake slower moving vehicles, the passing of which must be accomplished on lanes regularly used by opposing traffic. If passing is to be accomplished with safety, the driver should be able to see a sufficient distance ahead, clear of traffic, to complete the passing maneuver without cutting off the passed vehicle in advance of meeting an opposing vehicle appearing during the maneuver. When required, a driver can return to the right lane without passing if he sees opposing traffic is too close when the maneuver is only partially completed. Many passings are accomplished without the driver seeing a safe passing section ahead, but design based such maneuvers does not have the desired factor of safety. Because many cautious drivers would not attempt to pass under such conditions, design on this basis would reduce the usefulness of the highway.Passing sight distance for use in design should be determined on the basis of the length needed to safely complete normal passing maneuvers. While there may be occasions to consider multiple passings, where two or more vehicles pass or are passed, it is not practical to assume such conditions in developing minimum design criteria. Instead, sight distance is determined for a single vehicle passing a single vehicle. Longer sight distances occur in design and these locations can accommodate an occasional multiple passing.Sight distance for multilane highways It is not necessary to consider passing sight distance on highways or streets that have two or more traffic lanes in each direction of travel. Passing maneuvers on multilane roadways are expected to occur within the limits of each one-way traveled way. Thus passing maneuvers that require crossing the centerline of four-lane undivided roadways or crossing the median of four-lane divided roadways are reckless and should be prohibited.Notes1Conversely，it normally is of little practical value to provide passing sight distance on two-lane urban streets or arterials 相反地，通常在双车道城市道路或市区干线上提供超车视距实际价值不大。2The minimum sight distance available on a roadway should be sufficiently long to enable a vehicle traveling at or near the design speed to stop before reaching a stationary object in its path 道路上应有的最小视距应该足以使那些以等于或接近设计速度行驶的车辆能在到达行驶路线上的障碍物之前停下来。3However，these distances are often inadequate when drivers must make complex or instantaneous decisions, when information is difficult to perceive，or when unexpected or unusual maneuvers are required 但是，当驾驶员必须进行复杂或瞬间的判断，信息难以感觉到，或当需要进行出乎意外的或不同寻常的操纵时，上述停车距离往往是不足的。 4Decision sight distance is the distance required for a driver to detect an unexpected or otherwise difficult-to-perceive information source or hazard in a roadway environment that may be visually cluttered，recognize the hazard or its threat potential，select an appropriate speed and path，and initiate and complete the required safety maneuver safely and efficiently 判断视距是指驾驶员所需的这样一个距离：在可能引起视觉混乱的道路环境中发觉一个意外的或难以察觉的信息源或危险，或发觉其潜在的威胁，从而选择适当的速度和路线，并安全、有效地开始与完成所需的安全操作。 5Thus passing maneuvers that require crossing the centerline of four-lane undivided roadways or crossing the median of four-lane divided roadways are reckless and should be prohibited 因此，跨越无分隔设施的四车道道路中线或跨越四车道道路中央分隔带的超车运行是冒险的，应予以禁止。 Unit 9 Roundabout Intersections Text Where traffic flows are small then the control of traffic movements at intersections may be achieved by priority control. On the continent of Europe nearside priority is used where vehicles give way to traffic approaching from the right while in Australia off-side priority is used.As traffic flows increase delays with priority control become excessive and at high levels of flow grade-separated junctions are necessary. A junction of this form is however extremely expensive: in addition, land requirements are great and in urban and suburban areas interference with pedestrian flow can be considerable. For these reasons at-grade intersections either of the roundabout or signal control type are extremely important in urban areas. The characteristics of these junction types have been described by Millard and a consideration of these characteristics will usually determine which type of junction is appropriate. They include:In urban situations land requirements are usually the deciding factor. If this is so, it will be found that the land required for a large island roundabout is greater than that needed for traffic signal control. This is especially true if flows on one pair of arms are low. On the other hand, if land purchase is necessary, it is often easier to acquire corner sites necessary for a roundabout than the long narrow strips needed when parallel widening of traffic signal approaches is carried out.Both conventional and mini-roundabouts have difficulty in dealing with unbalanced flows, especially during peak hours when the traffic entering on an arm is considerably greater than the traffic leaving by it. In such situations there is frequently a shortage of gaps in the circulating stream and, under off-side priority role, delays may become excessive.Right-turning vehicles (left-hand rule of the road) cause difficulties with signal control when their numbers are large. Either late start or early cut-off facilities or a special phase must be provided causing reduced overall capacity at the junction. In such circumstances roundabouts offer advantages.Traffic signal control has difficulty in dealing with three-way junctions, especially where the flows are balanced. To a lesser extent this is true of junctions with five or more approaches.Roundabouts have been commonly used in central city areas where traditionally they were used to resolve traffic and pedestrian conflicts in the large open squares which existed in the early part of the twentieth century. The central island frequently covered a large area of the square and was utilized for ornamental flower beds whilst traffic circulated around the surrounding carriageway. Increasing traffic demand and the pressure to allow pedestrians to cross the carriageway at surface levelhave resulted in many of these roundabouts being converted to signal control so that more positive control over traffic movements on an area wide basis may be exercised. In suburban areas, roundabouts are frequently found at the intersection of radial and ring type roads where they are subjected to peak hour traffic demands due to commuter flows.Roundabouts are also used on rural roads where traffic flows or the road type do not justify the provision of a grade-separated intersection. In these situations speeds are high on the approaches to roundabouts and safety is an important consideration.Roundabouts deal efficiently with traffic movements when there are three or four arms. With three arms, and well balanced flows a roundabout is considered to be more efficient than signal control. When the number of arms exceeds four then direction signing and driver comprehension become difficult. With many approach arms the diameter of the roundabout increases, leading to possible higher circulating speeds and consequent safety problems.In addition to the resolution of traffic conflicts, roundabouts are employed where there is a significant change in road type, a change from rural to urban conditions or when a significant change in road direction is required.In current United Kingdom practice there are three basic types of roundabout: normal roundabouts, mini roundabouts and double roundabouts. These are variations of these types to form ring junctions, grade-separated roundabouts and signalized roundabouts.The Department of Transport defines a normal roundabout as one which has a one-way circulator carriageway around a kerbed central island 4m or more in diameter and usually with flared approaches to allow multiple vehicle entry.Mini roundabouts are defined as a roundabout having a one-way circulatory carriageway around a flush or a slightly raised circular marking less than 4m in diameter and with or without flared approaches. They have been widely used in urban areas where the speed limit does not exceed 30mph. Physical deflection of vehicle paths to the left, an important factor in roundabout safety, may be difficult in urban junctions with fixed kerb lines and in these circumstances road markings should be used to induce some vehicle deflection. The circular marking varies in diameter from 1 to 4m diameter and is domed to a maximum height of 125mm. If space within the junction is very limited then the central island will be frequently run over by larger vehicles and in these cases the island is normally flush with the road surface. The Department of Transport advise that pedal cyclists may experience difficulty and if there are a substantial number of cyclists passing through the junction then signal control may be preferable.A double roundabout is defined as an individual junction with two normal or mini roundabouts either contiguous or connected by a central link road or kerbed island. It is considered that this form of junction will have advantages in the following circumstances: improving an existing staggered junction where it avoids the need to realign one of the approach roads, unusual junctions such as scissors junctions, joining two parallel routes separated by a river, railway or motorway, existing crossroads where opposing right-turning movements can be separated overloaded single roundabouts, and junctions with more than four entries.A solution to the problem of large roundabouts where entries are approaching capacity may sometimes be found by conversion to a ring junction where the usual clockwise circulation of vehicles around a large island is replaced by two-way circulation with three-arm mini roundabouts at the junction of each approach arm with the circulatory carriageway. Because of the two-way flow on the circulatory carriageway and the need for circulating vehicles to give way, adequate signposting is essential for efficient operation.Roundabout control is also utilized in grade separated intersections in the form of two-bridge roundabouts and dumb-bell roundabouts.When roundabouts are being designed it is usual to add at least one additional lane at the entry of the approach roads to the circulating area with a maximum addition of two lanes and a maximum entry width of four lanes. When only low flows are predicted in the future then widening may not be considered necessary but a minimum of two lanes in an entry is desirable. The angle at which vehicles enter the circulating area is of considerable importance in the operation of a roundabout. If vehicles enter at an angle approaching 90to the circulating flow then vehicles are liable to stop qu
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